The invention concerns a polyurethane embedding material, based upon castor oil as well as further hydroxyl compounds, in particular trimethylol propane and aliphatic diisocyanates, its production and its use, particularly as embedding material for membranes.
Numerous embedding materials based upon polyurethanes are already known. Thus, for example, German Pat. No. 28 12 197 describes an embedding material for which, initially, a pre-adduct is produced from castor oil, trimethylol propane and a 2- to 4-fold excess of NCO-groups of an aromatic diisocyanate per equivalent OH-groups. This pre-adduct is then hardened with a mixture of castor oil and trimethylol propane, or even with castor oil alone. The so-prepared embedding material is suitable, in particular, for the embedding of membranes. Such embedded membranes have proved to be very useful, and have even been employed in various medical areas of use.
Under hydrolysis conditions, however, polyurethanes which are constructed from aromatic diisocyanates form, to a slight extent, aromatic amine compounds, which are suspected of possessing mutagenic or carcinogenic activity. For this reason, several countries have enacted very strict regulations for medically-employed arrangements, so as to have available embedding materials for the production of which only a very small amount, preferably no, aromatic compounds have been employed. For the preparation of medical devices such as modules for dialysis, hemofiltration, plasmaphoresis and the like, embedding materials are required which are resistant, i.e., they do not decompose or give off toxic compounds upon employment, which in other respects are bio-compatible, particularly blood-compatible and thereby are not physiologically objectionable.
Attempts have already been made to completely or partially substitute the aromatic diisocyanate upon production of embedding materials based upon polyurethane. Thus, for example, D.E.-O.S. 27 32 182 describes a stabilized pre-polymerizate for which the aromatic diisocyanate, 4,4'-diphenylmethane diisocyanate is substituted in part by a series of aliphatic diisocyanates, among others by isophoron diisocyanate. The materials described in this reference, are not, however, very hydrolysis stable, particularly through the presence of a polyester component. Moreover, biuret can be employed in the production of this embedding material, which makes it unsuitable for medical purposes. These embedding materials are thus predominantly employed for casting, filling, embedding and encapsulation of electrical components.
D.E.-A.S. No. 28 29 557 describes storage-stable free isocyanate group-containing pre-polymers based upon polyurethanes which still contain iron complex of acetyl acetone and/or acetyl acetic acid alkyl ester as catalyst. These pre-polymers can be employed, e.g., as lacquer solution, which hardens upon the working-in of water. The majority of pre-polymers employed in technical fields are suitable for medical purposes.
D.E.-O.S. No. 27 49 491 recommends numerous aromatic diisocyanates for the production of embedding materials. Indeed even aliphatic diisocyanates are mentioned there in general, although aromatic diisocyanates are preferably employed as building blocks for the production of these polyurethanes. As cross-linking agent, the reference mentions hydroxyl group-displaying esters of aliphatic carboxylic acids with at least 12 carbon atoms. However, difficulties occur upon the working up of such embedding materials and, moreover, the described compositions are physiologically hazardous.
D.E.-O.S. 31 49 527 likewise describes polyurethane masses which are supposed to be suitable as embedding material for hollow fibers in dialyzators. They have been prepared on the basis of castor oil, multi-functional hydroxyl compounds, as well as dissolving agents such as hydroxyl group-containing partial esters of long-chain carboxylic acids with co-employment of organo-tin compounds as catalysts and with the use of customary diisocyanate. However, the polyurethanes described in this Offenlegungschrift are for a number of reasons hazardous from a physiological point of view. Thus, preferably, aromatic diisocyanates are recommended for construction of the polyurethane. The tin/sulphur compounds mentioned therein as catalysts can act toxically. There are also objections against the employment of zeolites in parts which come into contact with blood. Finally, the employment of dissolving agents for embedding materials that are to be used in medical fields is dangerous.
The plurality of patent disclosures shows that it is extremely difficult to manufacture embedding materials with specific characteristics suitable for determined purposes of use. With many techniques it is necessary to insert great amounts of catalysts, which can have toxic effects and therefore meet with objections, particularly in the medical fields. On the other hand, the construction of polyurethanes is complicated, and through the insertion of higher-molecular substances, such as polyethers, polyesters, semi-esters and the like, the mechanical characteristics such as hardness are disadvantageously influenced.
Although numerous embedding materials based upon polyurethanes are already known, there still exists a need for improved embedding materials, as well as simple to perform processes for the production thereof, which do not possess the above-described disadvantages.